With 2 different objects, its not possible to control as each object will work separately.
I_TCODE will be called when you click on IW31 in SAP easy screen. I_AUART will work at the header but with Order type & Plant.
As there is no check in point Order header, its possible only at order save event. Yes, I understand, after entering all the details in PM Order & giving error message to user about this error can make them uninterested. But I think, there is no option for header for this check.
Check below Wiki document for different enhancement points in PM. If something suits you other than before mentioned exit, please update.
